RSC Advances

PAPER

CO2 reforming o

aDepartment of Chemical Engineering, Fac

Teknologi Malaysia, 81310 UTM Johor BahrbInstitute of Hydrogen Economy, Universiti

Bahru, Johor, MalaysiacDepartment of Chemistry, Faculty of Scien

UTM Johor Bahru, Johor, Malaysia

† Electronic supplementary informa10.1039/c5ra04320d

Cite this: RSC Adv., 2015, 5, 37405

Received 11th March 2015Accepted 20th April 2015

DOI: 10.1039/c5ra04320d

www.rsc.org/advances

This journal is © The Royal Society of C

f CH4 over Ni/mesostructuredsilica nanoparticles (Ni/MSN)†

S. M. Sidik,a A. A. Jalil,*ab S. Triwahyono,c T. A. T. Abdullahab and A. Ripinab

The development of supported Ni-based catalysts for CO2 reforming of CH4 was studied. Ni supported on

mesostructured silica nanoparticles (MSN) and MCM-41 were successfully prepared using an in situ

electrochemical method. The N2 physisorption results indicated that the introduction of Ni altered

markedly the surface properties of MCM-41 and MSN. The TEM, H2-TPR and IR adsorbed CO studies

suggested that most of the Ni deposited on the interparticles surface of MSN have higher reducibility

than Ni plugged in the pores of MCM-41. Ni/MSN showed a higher conversion of CH4 at about 92.2%

compared to 82.6% for Ni/MCM-41 at 750 �C. After 600 min of the reaction, Ni/MCM-41 started to

deactivate due to the formation of shell-like carbon which may block the active sites and/or surface of

catalyst, as proved by TEM analyses. Contrarily, the activity of Ni/MSN was sustained for 1800 min of the

reaction. The high activity of Ni/MSN was resulted from the presence of greater number of easily

reducible Ni on the surface. In addition, the large number of medium-basic sites in Ni/MSN was capable

to avoid the formation of shell-like carbon that deactivated the catalyst, thus increased the stability

performance. The results presented herein provide new perspectives on Ni-based catalysts, particularly

in the potential of MSN as the support.

Introduction

CO2 reforming of CH4 has advantages over existing industrialprocesses for synthesis gas production, from both environ-mental and industrial perspectives. CO2 reforming of CH4 notonly converts CO2 and CH4, which are recognized as undesir-able greenhouse gases, into more desirable products, but alsooffers an alternative route for the production of syngas with alow H2/CO molar ratio.1 During recent decades, considerableefforts have been focused on the development of catalysts forCO2 reforming of CH4 in order to achieve high catalytic activitywith a high resistance to coking. Although some formulationsbased on noble metals are very stable and active, Ni-basedcatalysts present the best balance between economic viabilityand catalytic performance.2 However, the major obstacleencountered in the industrialization of CO2 reforming of CH4

over Ni-based catalysts, is the rapid deactivation of the catalyst,which is mainly caused by coke accumulation on the active sitesand sintering of the active metallic phase.3 Therefore,

ulty of Chemical Engineering, Universiti

u, Johor, Malaysia

Teknologi Malaysia, 81310 UTM Johor

ce, Universiti Teknologi Malaysia, 81310

tion (ESI) available. See DOI:

hemistry 2015

improvement of the catalyst stability remains an importantchallenge.

A strategy that can be used to address coke deposition is toenhance the metal dispersion and to increase the number ofbasic sites. Sintering of metallic Ni particles on conventionalsupports, such as Al2O3 and SiO2, is inevitable at high reactiontemperatures in the CO2 reforming of CH4.4 This is due to therelatively large Ni particle size (more than 10 nm), which is notsmall enough to achieve high anti-sintering and coke-resistantproperties.5 Recently, we reported a new method for the prep-aration of metal nanoparticles using a simple electrochemicalmethod and its successful application in the synthesis ofvarious types of drug precursors.6,7 By applying the corre-sponding method, Zn- and Ni-promoted zeolite catalysts led tothe efficient isomerization of petrochemical products.8,9

The incorporation of Ni particles into mesoporous silicasupports has been shown to provide a high dispersion of Niparticles, and hence has improved the stability of Ni catalysts inCO2 reforming of CH4.10,11 In recent years, mesoporous silicananoparticles (MSN) with a highly ordered mesostructure, a highsurface area and a large pore volume have been effectivelyutilized in the elds of adsorption, drug delivery and catal-ysis.5,12,13 One of the important features of MSN compared toother mesoporous silica is their interparticles textural porositythat gives rise to surface area and basicity. This material presentsan opportunity for the design of highly accessible active sites onthe surface of interparticles voids, which contribute to a betterreaction between the reactant and catalyst. However, no studies

RSC Adv., 2015, 5, 37405–37414 | 37405

RSC Advances Paper

of CO2 reforming of CH4 have been reported to date using MSNcatalyst, notwithstanding some reports over mesoporous silicaimpregnated with Ni catalysts. These features led to an interest inexploring the potential of MSN as a new support material in CO2

reforming of CH4.In this study, a highly dispersed Ni nanoparticles supported

on mesostructured silica nanoparticles (Ni/MSN) was synthe-sized using an electrochemical method. Ni-supported on mes-oporous MCM-41 (Ni/MCM-41) was also synthesized using thesame technique to be used as part of a comparative study. Bymeans of various characterization techniques, the physico-chemical properties of the catalysts were investigated. Thecatalytic performances for CO2 reforming of CH4 were evaluatedand compared, and the relationships between catalytic behaviorand property of catalysts have been established. In particular,the catalyst deactivation aspect has been discussed in detail.

ExperimentalSynthesis of MSN and MCM-41

MSN was prepared by co-condensation and sol–gel method aspreviously reported.12 In brief, the cetyltrimethylammoniumbromide (CTAB, Merck), ethylene glycol (EG, Merck) and ammo-nium (NH4OH, QRec) solution were dissolved in 700mL of doubledistilled water with the following mole composition ofCTAB : EG : NH4OH : H2O¼ 0.0032 : 0.2 : 0.2 : 0.1. Aer vigorousstirring for about 30 min at 50 �C, 1.2 mmol tetraethylorthosili-cate (TEOS, Merck) and 1 mmol 3-aminopropyl triethoxysilane(APTES, Merck) were added to the clear mixture to give a whitesuspension solution. This solution was then stirred for another 2h at 80 �C, and the as-synthesized MSN sample collected bycentrifugation at 20 000 rpm. The as-synthesized MSN were driedat 110 �C overnight and calcined at 550 �C for 3 h to remove theimpurities. The amino group and surfactant were successfullyremoved aer calcination, as evidenced by the absence of aminoand methyl groups in the FTIR results.

MCM-41 was prepared by hydrothermal method based on areport by Iwamoto et al. by using dedocyltrimethylammoniumbromide C12H25N(CH3)3Br, colloidal silica and water.14 Thismixture was homogenized at room temperature by stirring. Theresultingmixture was loaded into a Teon bottle in an autoclaveand statically heated at 140 �C for 44 h. The product was washedwith deionized water and dried at 110 �C overnight. Finally, toremove remaining template ions, the sample was heated in airat a heating rate of 5 �C min�1 to 150 �C and then at a rate of0.2 �C min�1 to 600 �C, and held at 600 �C for 6 h.

Introduction of Ni on MSN and MCM-41

In this study, Ni modied MSN (Ni/MSN) was prepared by in situelectrochemical method. Platinum (Pt, Nilaco) and nickel (Ni,Nilaco) plates (2 cm � 2 cm) were used as anode and cathode,respectively. Then, 30 mL of N,N-dimethylformamide (DMF,Merck) solution was added to a one-compartment cell con-taining tetraethyl ammonium perchloride (TEAP), naphthalene(Fluka) and MSN. Naphthalene was used as a mediator toproduce radical anions, which then reduced the Ni cations to

37406 | RSC Adv., 2015, 5, 37405–37414

give much smaller Ni nanoparticles.7 The electrolysis was con-ducted under continuous stirring at a constant current densityof 480mA cm�2 and 0 �C under a N2 atmosphere. The desired Nicontent supported on the MSN and the time required for elec-trolysis was calculated based on the Faraday's law of electrolysis(refer ESI Table S1†), as shown in the following equation,

n ¼�It

F

��1

z

�(1)

where n is the number of moles of Ni, I is the constant current ofelectrolysis (A), t is the total time the constant current wasapplied (s), F is Faraday's law constant (96 487 Cmol�1), and z isthe valence number of ion of the substance (electron transferredper ion).

Aer electrolysis, the mixture was heated up at 85 �C toremove the remaining solvents before being dried overnight at110 �C. Finally, the sample was calcined at 550 �C for 3 h to givea grey-colored Ni/MSN catalyst. For comparative study, Ni/MCM-41 was synthesized under identical experimental condi-tions as described above. Ni/MCM-41 was also prepared by wetimpregnation method for comparison study. The aqueousnickel nitrate (Ni(NO3)2$6H2O) was impregnated on theMCM-41 at 60 �C, and was then dried in an oven at 110 �Covernight before calcination in air at 550 �C for 3 h.

Characterization

The crystalline structure of the catalysts was determined withX-ray diffraction (XRD) recorded on powder diffractometer(Bruker Advance D8, 40 kV, 40 mA) using a Cu Ka radiationsource in the range of 2q ¼ 1.5–80�. N2 adsorption–desorptionisotherms were used to determine the textural properties at liquidnitrogen temperatures using a Micromeritics ASAP 2010 instru-ment. The Brunauer–Emmett–Teller (BET) and Non LocalizedDensity Functional Theory (NLDFT) methods were used tocalculate the surface area and pore distribution of the catalysts,respectively. Prior to measurement, all of the catalysts were out-gassed at 110 �C for 3 h before being subjected to N2 adsorption at�196 �C. The elemental analyses of Ni content in a catalyst weredetermined with Agilent Technologies 4100 MP-AES. 29Si MASNMR spectra were obtained on a Bruker Solid NMR (JEOL 400MHz) spectrometer. The 29Si MAS NMR signal of tetramethylsi-lane (TMS) was used as the chemical-shi reference. The spectrawere recorded using 4 ms radio frequency pulses, a recycle delay of60 s and spinning rate of 7 kHz using a 4 mm zirconia samplerotor. Fourier Transform Infrared (FTIR) measurements werecarried out using Agilent Technologies Cary 640 FTIR Spectrom-eter. The catalyst was prepared as a self-supported wafer andplaced in a high-temperature stainless steel cell with CaF2windows. Then, the catalyst was reduced in H2 stream (15 mLmin�1) at 400 �C for desired reduction periods, followed by out-gassing at 400 �C for 1 h. The adsorption of pyrrole was carriedout at room temperature for 30min, followed by desorption at 50,100 and 150 �C for 15 min, respectively. For the CO adsorptionprocess, the reduced catalyst was exposed to 10 Torr of CO for 30min. All spectra were recorded at room temperature with aspectral resolution of 5 cm�1 with ve scans. In order to compare

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Paper RSC Advances

the surface coverage of the adsorbed species between differentwafer thicknesses, all spectra were normalized using the overtoneand combination vibrations of the lattice between 2100 and 1550cm�1.10H2-TPR experiments were carried out usingMicromeriticsChemisorb 2720 Pulse Chemisorption in 10% H2/Ar at 10 �Cmin�1. H2 chemisorption was measured to investigate the Nidispersion and Ni surface area of the catalysts. Prior to thechemisorption, 30 mg of the catalyst was reduced with pure H2

(20 mL min�1) at 900 �C for 1 h. The amount of hydrogen uptakewas determined by injecting mixed gas (10% H2/Ar) periodicallyinto the reduced catalyst. The Ni dispersion and Ni surface areawere calculated by assuming that one hydrogen atom occupiesone Ni atom. The amount of coke deposits was determined withthermogravimetric-differential thermal analysis (TG-DTA) using aPerkin Elmer TGA7 Thermogravimetric Analyzer. Temperaturewas programmed from ambient temperature to 900 �C at aheating rate of 10 �Cmin�1 under N2 ow. Transmission electronmicroscopy (TEM) was carried out using a JEOL JEM-2100Fmicroscope. The samples were ultrasonically dispersed inacetone and deposited on an amorphous, porous carbon grid.

Fig. 1 (A) Low- and (B) wide-angle XRD patterns of MSN and MCM-41type catalysts.

Catalytic testing

The catalytic CO2 reforming of CH4 was conducted in acontinuous ow microcatalytic reactor at atmospheric pressureand in a temperature range of 500–800 �C. The gas hourly spacevelocity (GHSV) was kept constant at 15 000mL h�1 gcat.

�1. Priorto the catalytic testing, 0.2 g of catalyst was reduced in a H2 owof 50 mLmin�1 for 3 h at 900 �C. The feeding gas ow rate to thereactor was set at 50 mL min�1, with a gas molar ratio of CH4-: CO2 : N2 ¼ 1 : 1 : 1, as N2 was used as a carrier gas. Theeffluent gas was analyzed with an online 7820A gas chromato-graph (Agilent Technologies) equipped with a Carboxen 1010column (Sigma-Aldrich) and a thermal conductivity detector(TCD). The CH4 and CO2 conversion was reported as percentageof CH4 and CO2 converted. The selectivity of the products to H2,SH2

and CO, SCO were calculated based on the equation describeas follows:

SH2¼ ½H2�out

2�½CH4�in � ½CH4�out

��100 (2)

SCO ¼ ½CO�out�½CH4�in � ½CH4�out�þ �½CO2�in � ½CO2�out

��100 (3)

where [H2]out and [CO]out are the molar concentration of H2 andCO produced during the reaction, respectively. Finally, productdistribution ratio, H2/CO was calculated based on eqn (4).

H2

CO¼ SH2

SCO

(4)

Results and discussionCharacterization of the catalysts

XRD is one of the most important techniques for character-izing the structure of ordered materials. Low- and wide-angleXRD patterns of MSN and MCM-41 type catalysts are shown

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in Fig. 1. Fresh MSN and MCM-41 showed three peaks at 2.35,4.05, and 4.75� (Fig. 1A), corresponding to the (100), (110), and(200) planes, reecting the ordered structure of the 2Dhexagonal space group (p6mm).15 The introduction of Ninotably decreased the intensity of all peaks and almost elim-inated the (110) and (200) peaks of the Ni/MSN, indicating areduction in the long-range order of the hexagonal arrange-ment. In addition, the (100) diffraction for Ni/MSN and Ni/MCM-41 shied to lower angles compared to the fresh MSNand MCM-41 samples, demonstrating an increase in the cor-responding unit cell parameters. As shown in Table 1, the unitcell parameters of both MSN and MCM-41 increased with theaddition of Ni, from 4.34 and 4.54 nm to 4.46 and 4.61 nm,respectively. The expansion of the unit cell might be due to thesubstitution of silicon ions (Si4+, ionic radius ¼ 0.41 A) withthe larger Ni ions (Ni2+, ionic radius ¼ 0.69 A) in the silicaframework.16 In the wide-angle XRD patterns of Ni/MSN(Fig. 1B), two new peaks were observed at 2q ¼ 43.1 and62.8� corresponding to the respective (200) and (220) planes ofcubic NiO species (JCPDS 78-0643). No NiO crystals wereobserved on Ni/MCM-41, indicating that the Ni was homoge-nously dispersed on the support or the crystallite sizes weresmaller than the detection limit of XRD.17 A similar phenom-enon was also observed for metal loaded SBA-15 mesoporousand alumina.15,18

The N2 adsorption–desorption isotherms and pore sizedistributions of the MSN and MCM-41 type are shown in Fig. 2.Both MCM-41 and MSN exhibited type IV isotherm, which is atypical feature for this type of mesoporous material, accordingto the IUPAC classication.19 TheMCM-41 showed no hysteresisloop, indicating reversible pore lling and emptying. Mean-while, the MSN exhibited a type H4 hysteresis loop, suggesting auniform slit shape pores. In spite of having similar isothermtype, there were pronounced differences in their pore struc-tures. A sharp increase in nitrogen uptake was observed in therelative pressure range of 0–0.03 and 0.3–0.4 for MCM-41,showing the presence of both micropores and mesopores.Similarly, the MSN also displayed a clear and sharp adsorption

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Table 1 Textural properties of fresh and Ni-modified catalysts

Catalysts

Surface area [m2 g�1]Pore volume[cm3 g�1] Pore sizec [nm] Unit celld [nm] Ni dispersione [%] Ni surface areae [m2 gcat.

�1]BET Microporea Mesoporeb

MSN 894 105 789 0.977 2.58 4.34 n.a n.aNi/MSN 772 25 747 1.001 3.17 4.46 10.5 36.1MCM-41 877 101 776 0.630 3.78 4.54 n.a n.aNi/MCM-41 337 270 67 0.279 2.70 4.61 11.7 40.2

a Micropore area was obtained from t-plot method. b Mesopore area ¼ Surface area � micropore area. c Average pore size was calculated obtainedfrom Non Localized Density Functional Theory (NL-DFT). d The unit cell parameter was calculated from unit cell ¼ 2d100=

ffiffiffi3

p. e Determined from

H2 chemisorption.

Fig. 2 (A) N2 adsorption–desorption isotherms and (B) pore sizedistributions of MSN and MCM-41 type catalysts.

Fig. 3 TEM images and EDX analyses of (A) Ni/MSN and (B) Ni/MCM-41.

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step at a relative pressure in the 0–0.03 range. One of theadditional features of MSN is their capillary condensation athigher relative pressure of 0.85–0.95, indicating the presence oflarger pores that resulted from interparticles voids. When Niwas incorporated into the MCM-41, the pore lling step at anintermediate relative pressure disappeared. However, theaddition of Ni into the MSN decreased the microporosity andslightly increased the interparticles mesopores. Both MSN andMCM-41 showed a narrow pore distribution (Fig. 2B) in the 2–5nm range. The texture properties of each of the catalysts aresummarized in Table 1. The introduction of Ni into MCM-41signicantly decreased the BET and mesopores surface area,and simultaneously increased micropores surface area. More-over, the average pore size and pore volume of MCM-41 mark-edly decreased, suggesting the accumulation of Ni particlesinside the pore mouth and partially closed the mesopores.Lovell et al. reported the formation of smaller NiO can belocated in the MCM-41 pores and evidenced by the decrease inpore volume.20 In contrast, a slight decrease in the BET specicsurface area was observed for the Ni/MSN catalyst especially inmicropore surface area compared to that of the MSN support.Also, an increase in the pore volume and average pore size wasnoted, which may be due to the tendency of the Ni species todeposit on the interparticles void of the MSN and blocking themicropores.21

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The TEM images of Ni/MSN and Ni/MCM-41 are shown inFig. 3. Some darker gray nanoparticles on the TEM images couldbe determined as Ni particles. The EDX analysis recorded on thered square area further conrmed that the nanoparticles are Ni.The average particles size of Ni on Ni/MSN is in the range of 3–8nm, larger than that on Ni/MCM-41 (2–5 nm). From the images,Ni particles were distributed on the interparticles surface ofMSN and mostly accumulated at the center of Ni/MCM-41. Inaccordance to the N2 physisorption results, it is suggested thatmost of the Ni nanoparticles lled and/or blocked the pores ofMCM-41, whereas most of the Ni nanoparticles were located onthe interparticles surface of the MSN.

The FTIR spectra of MSN and MCM-41 type catalysts in theregion between 1400 and 400 cm�1, are illustrated in Fig. 4A.For MSN and MCM-41, the bands at 1056 and 797 cm�1 wereassigned to the asymmetric and symmetric stretching vibra-tions of Si–O–Si in the framework, respectively. Other bandswere observed at 956 and 460 cm�1, corresponded to theexternal Si–OH groups and Si–O–Si bending, respectively. Theaddition of Ni shied the Si–O–Si peaks to lower wavenumbersand simultaneously decreases the external Si–OH and Si–O–Sibending groups. This is suggested that desilication processesoccurred and that there was a possible interaction between theNi species and the Si–O–Si group.22 The shis may be attributedto the increase in the mean Si–O distance in the walls of thecatalysts, caused by the substitution of silicon with larger Niatoms.16 A similar observation was also reported for thesubstitution of silicon ions in MCM-41 with manganese (Mn2+)and zirconium (Zr4+) ions, which resulted in the shi of the anti-symmetric Si–O–Si vibration bands to lower wavenumbers.22

However, no obvious band was observed in the region of 962–

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Fig. 4 (A) IR spectra of KBr and (B) activated of (a) MSN, (b) Ni/MSN, (c)MCM-41 and (d) Ni/MCM-41 catalysts. Fig. 5 29Si MAS NMR spectra of MSN and MCM-41 type catalysts.

Green curves represent the Gaussian curve-fitting spectra.

Paper RSC Advances

967 cm�1, which corresponds to the vibration of the Si–O–Nibond. It may be because of an overlap with the characteristicstretching frequencies of siliceous materials in this region.23

Fig. 4B shows the FTIR spectra of activated (evacuated at400 �C for 1 h prior to IR measurement) MSN and MCM-41type catalysts in the stretching region of the hydroxyl groupsat 3200–3800 cm�1. The sharp peak at 3740 cm�1 is assigned tothe terminal silanol groups located on the external surface ofthe parent mesoporous.24 Another broad hydroxyl group onstructural defects and/or vicinal hydroxyl groups centered at3550 cm�1 were also observed. The introduction of Ni nano-particles onto the MSN and MCM-41 decreased the intensity ofthe terminal silanol, structural defects and vicinal hydroxylgroups, suggesting a possible interaction between the Ninanoparticles with the Si atom through the O atom. Sapaweet al. also found that the decreased in intensity of the hydroxylgroups as the metal loading increased, suggesting increasedformation of Si–O–Zr bonds in the catalysts.25

29Si MAS NMR spectroscopy offers an excellent opportunityto monitor structural changes and the diversity of silicon envi-ronments in the framework of MSN and MCM-41 type catalysts(Fig. 5). The 29Si NMR spectra of MSN and MCM-41 consists ofthree peaks at �92 ppm, �101 ppm, and �110 ppm, which areattributed to the (^SiO)2Si, (^SiO)3Si, and (^SiO)4Si structuralgroups, respectively.26 The presence of Ni caused a slightdecrease in the intensity of the (^SiO)4Si, suggesting theextraction of framework silicon ions from MSN and MCM-41through the desilication process during the electrolysisprocess. It is well known that, in alkaline solution, silicon willbe selectively removed from its framework. For instance, Qinet al. observed a gradual decrease in the intensity of the Si(nAl)peaks with high Si content (n ¼ 0,1) of NaY zeolite aer alkalitreatment.27 Similar results were reported by Jusoh et al., whostated that the introduction of ZnO into MSN in an alkalinemedium reduced the intensity of the (^SiO)4Si peak andproduced two new peaks for (^SiO)3Si and (^SiO)2Si.28 The(^SiO)3Si peaks were diminished and the (^SiO)2Si peakswere, infact, featureless for both Ni/MSN and Ni/MCM-41,demonstrating that both free and geminal silanol groups are

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the active sites for the interaction between the Ni nanoparticlesand the supports. These results are in line with FTIR results,which showed interactions between Ni nanoparticles andhydroxyl groups as well as framework silica in the catalysts. Inthe surfacemodication of MCM-41 with trimethylchlorosilane,Zhao et al. reported a decrease in the intensity of the (^SiO)3Siand (^SiO)2Si peaks, suggesting that both sites are responsiblefor active silylation.29

To investigate the reducibility of both Ni/MSN and Ni/MCM-41, H2-TPR experiments were carried out. The TPR proles ofNiO, Ni/MSN and Ni/MCM-41 were depicted in Fig. 6.

For NiO, there were three reduction peaks at 320, 370 and391 �C, which can be ascribed to black Ni2O3, large and smallNiO, respectively.20 In the case of Ni/MSN and Ni/MCM-41, thereduction peak of Ni2O3 shied to 346 �C, which is due to weakinteraction between Ni2O3 and MSN. Accordingly, a new broadshouldered peak of hydrogen consumption was observed at491 �C, which is likely resulted from the reduction of Ni speciesthat strongly interacted with the supports.30 By comparing thereduction peak at 491 �C, Ni/MCM-41 showed relatively higherhydrogen consumption compared to Ni/MSN, illustrating thatthe insertion of Ni species most readily deposited in the pore ofMCM-41 and resulted in lower reducible Ni species.

Fig. 6 H2-TPR profiles of NiO, Ni/MCM-41 and Ni/MSN catalysts.

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The metal surface area and metal dispersion for Ni/MSN andNi/MCM-41 catalysts are compiled in Table 1. From the H2

chemisorption results, the dispersion of metallic Ni and metalsurface area on MCM-41 are higher than those on MSN. Thedifferences are probably due to weaker interaction between Niand MSN, thus resulting in remarkable nucleation and aggre-gation of large Ni crystallites during the reduction. On the otherhand, the smaller Ni particles that are strongly interacted withthe MCM-41 prevent the agglomeration or sintering of the Ninanoparticles.

Infrared spectroscopy of adsorbed CO was also employed tostudy interaction between CO and reducible Ni in Ni/MSN andNi/MCM-41. IR spectra of CO adsorbed on reduced Ni/MSN andNi/MCM-41 are shown in Fig. 7. The spectra consist of threepeaks located at 2156, 2105, and 1859 cm�1, which wereassigned to different CO adsorption modes. Two peaks at 2156and 2105 cm�1 were attributed to physically adsorbed CO. Thepeaks at 1859 cm�1 could be assigned to the adsorption of COon Nio bridge sites (Ni2–CO) of the Ni (111) crystalline plane.31

Aer 1 h reduction on Ni/MSN, two new bands appeared at 2063and 2040 cm�1, which correspond to terminally bonded CO andadsorbed nickel tetracarbonyl [Ni(CO)4], respectively.32 For Ni/MCM-41, no signicant peak associated with the formation ofeither Ni]CO or Ni(CO)4 was observed. This is probably due toincomplete reduction of Ni nanoparticles present inside theMCM-41 pores that originated from the stabilization of the Nications in the silica framework.

To further investigate the degree of reducibility of Ni/MSNand Ni/MCM-41, both catalysts were subjected to differentreduction periods before CO adsorption. Aer reduction for 2 h,a new peak at 2040 cm�1 corresponding to Ni]CO, was noticedfor Ni/MCM-41. At prolonged reduction periods, an additionalpeak at 2063 cm�1 attributed to the formation of Ni(CO)4 wasalso observed. Meanwhile for Ni/MSN, a longer reduction didnot produce new peak; however, it did intensify the signals.

The difference in reduction period of Ni/MSN and Ni/MCM-41 could be reected the location of Ni nanoparticles, becauseNi located on the interparticles surface has a greater reducibilitythan Ni in the pore walls. In fact, Lim et al. found that theanchoring and partial occlusion of metallic clusters on the pore

Fig. 7 IR spectra of adsorbed CO on reduced Ni/MSN and Ni/MCM-41catalysts.

37410 | RSC Adv., 2015, 5, 37405–37414

walls were the main reasons for the stabilization of the catalystsagainst reduction.33 The H2-TPR and IR adsorbed CO indicatedthat Ni deposited on MSN and MCM-41 at different locations. Itis, therefore, suggested that the location of Ni is on the inter-particles surface of MSN. In contrast, the larger pore size ofMCM-41 allowed the Ni nanoparticles to accumulate inside thepores, for which a longer reduction period is needed in order toreduce the Ni nanoparticles.

Nature of the basicity

IR spectra of adsorbed pyrrole were obtained for fresh and Ni-modied catalysts to quantitatively measure the strength andthe relative amount of basic sites. Fig. 8A shows IR adsorbedpyrrole spectra in which a dotted line indicates the position ofN–H from pyrrole molecules in the gas phase which located at3530 cm�1 for all samples. The second band, indicated by anarrow in the gure is attributed to the perturbed N–H stretch ofpyrrole molecules interacting with the basic sites of theframework oxygen atoms. The H-donor properties of pyrroleallowed the formation of C4H4NH–O bridges with basic oxygen.The asterisk symbol (*) at 3410 cm�1 can be assigned to phys-isorbed pyrrole in a liquid-like state, in which the N–H group isinteracting with the p-system of another pyrrole molecule.34

The basic sites of the catalyst can be determined based onthe area under the main peak. A higher peak area indicates ahigher concentration of basic sites in the catalyst. Thus, thenumber of basic sites in fresh and Ni-modied catalystsincreased in the order of MCM-41 < MSN < Ni/MCM-41 < Ni/MSN, as shown in Fig. 8B. The incorporation of Ni nano-particles into the fresh supports slightly increased the numberof basic sites and the basic strength. Previously, Aziz et al. haveobserved a similar basicity in Ni/MSN and Ni/MCM-41 preparedusing the impregnation method.35 This observation may arisefrom the similar surface area between the catalysts, thusproviding an equally accessible site for pyrrole adsorption. Inthis study, it is worth mentioning that the number of surface

Fig. 8 (A) IR spectra of pyrrole adsorbed on reduced (a) MSN, (b) Ni/MSN, (c) MCM-41 and (d) Ni/MCM-41 when the catalysts wereexposed to 4 Torr of pyrrole at room temperature. (B) The shift value ofN–H stretching and peak area of basic site of reduced MSN and MCM-41 type catalysts. (C) Effect of desorption temperature on the relativeintensity of pyrrole adsorbed on reduced MSN and MCM-41 typecatalysts.

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Fig. 9 Effect of reaction temperature on (A) CH4 conversion, (B) CO2

conversion, (C) H2/CO ratio and (D) TOF values over Ni/MSN and Ni/MCM-41 catalysts.

Paper RSC Advances

basic sites provided by the Ni/MSN is slightly greater than Ni/MCM-41, which may resulted from higher surface area avail-able for pyrrole adsorption.

In addition, the higher intensity of hydroxyl groups in thestructural defects in Ni/MSN may contribute to its superiorbasicity.36 The IR band associated with N–H vibrations shiedto a lower wavenumber upon the interaction of pyrrole with thebasic sites. The strength of the basic sites present in the freshand Ni-modied catalysts was measured based on the shi ofthe N–H stretching.37 As depicted in Fig. 8B, it is suggested thatthe strength of the basic sites in the fresh and Ni-modiedcatalysts is in the order of Ni/MCM-41 (�65 cm�1) > Ni/MSN(�64 cm�1) > MCM-41 (�62 cm�1) > MSN (�60 cm�1). Eachof these values is slightly lower than the shi observed for theinteraction of a H atom in the pyrrole molecule with basicoxygen in an alkali metal supported zeolite (80 < Dv (N–H) < 150cm�1). Thus, it is suggested that the pyrrole molecule interactswith catalyst surfaces mainly through the aromatic ring, and theN–H group is assumed to be free or pseudo-free.38 The strengthof the basic sites in the catalyst can also be determined from theeffect of the outgassing temperature of the pyrrole adsorbedcatalysts. Fig. 8C displays the relative intensity of the main peakof fresh and Ni-modied catalysts at outgassing temperaturesbetween 30 and 150 �C. The intensity of the N–H band inter-acting with basic sites on the catalysts decreased monotonicallywith an increase in the outgassing temperature, which can beattributed to the weak interaction between the pyrrole speciesand the catalysts. Thus, this result suggested that the oxygen inthe framework of fresh and Ni-modied catalysts has low elec-tron donor ability, that is, it has medium basicity.

Catalytic testing

The catalytic activity of fresh and Ni-modied catalysts wasevaluated according to the degree of conversion of CH4 andCO2, the ratio of H2/CO and TOF of CH4 conversion at differentreaction temperatures. The effect of the reaction temperatureon the catalytic performances of Ni/MSN and Ni/MCM-41 isdepicted in Fig. 9. Generally, both CH4 (Fig. 9A) and CO2

(Fig. 9B) conversions increased with increasing reactiontemperature for all catalysts, reecting the endothermic char-acter of CO2 reforming of CH4. The reaction over bare MSN andMCM-41 catalysts showed lower catalytic activity (<5% of CH4

and CO2 conversion), indicating that metallic Ni is necessary forthe studied catalytic system.

The performances of Ni-modied catalysts were dependenton both the nature of the support materials and the reactiontemperature. When the reaction temperature was increased to600 �C, CH4 conversion for Ni/MSN reached 54.6% which nearly1.3 times higher than that of Ni/MCM-41. The conversion ofCH4 increased with the further temperature increase to 800 �Cin which the conversion reached 93.6 and 90.8% for Ni/MSNand Ni/MCM-41, respectively. In the range of temperaturestudied, the activity of Ni/MCM-41 prepared by electrolysismethod was slightly higher compared to the Ni/MCM-41prepared by conventional impregnation method (refer ESIFig. S1†). At 700 �C, the CH4 conversion of Ni/MCM-41 is similar

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to that reported by Lovell et al.16 for 2.5% Ni/MCM-41 preparedby impregnation method. This nding demonstrates the Ni/MCM-41 and Ni/MSN prepared by in situ electrochemicalmethod can produce an activity comparable and greater thanMCM-41 prepared by more conventional means, respectively.The activity of Ni/MSN and Ni/MCM-41 catalysts was alsocompared by turnover frequency (TOF) for CH4 conversionconsidering the dispersion of Ni particles as shown in Fig. 9D.The TOF was calculated based on the mole of CH4 converted permole of active Ni per second. It is worth noticing that the Nidispersion and metal surface area of Ni/MCM-41 were largerthan those of Ni/MSN due to the formation of smaller Niparticles. On the other hand, the activity of Ni/MSN was foundto be 1.2 times higher than Ni/MCM-41 at 750 �C. A signicanteffect of support is related to the difference of Ni particles size,since a so-called ‘structure-sensitive” reaction is inuenced bydispersion of metal on the support. Another possibility ofsupport effect may be due to a direct activation of CH4 or CO2 bythe support. Although no signicant activity was observed forfresh MSN, the use of MSN as a support for Ni particlesincreased the catalytic activity. This indicated a synergisticeffect of Ni and MSN on the catalytic activity for CO2 reformingof CH4. Although the exact reason is currently unclear, the useof MSN as a support is advantageous for CO2 reforming of CH4.

In case of CO2 conversion, the rate also increased parallel toan increase in the reaction temperature and started to decline atabove 750 �C probably due to coking from excess methanedissociation to hydrogen and coke.39 At 500 �C, CO2 conversionover Ni/MCM-41 was higher than that of Ni/MSN. From the H2-TPR analysis, Ni/MCM-41 exhibited lower reducibility than Ni/

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Fig. 10 Stability and regeneration study over Ni/MSN and Ni/MCM-41catalysts.

RSC Advances Paper

MSN. Therefore, reaction at the same feed ratio of CH4/CO2 andat low temperature, the Ni sites of Ni/MCM-41 are not capable toadsorbed CH4 as much as Ni/MSN, which resulted in theincreasing of CO2 adsorption and activation on the Ni/MCM-41surface. Therefore, a higher CO2 conversion can be observed forNi/MCM-41 compared to Ni/MSN.

In this study, Ni/MSN displayed a higher CH4 and CO2

conversions than Ni/MCM-41. This is likely to be caused by thepresence of more easily reducible Ni nanoparticles on theinterparticles surface of the catalyst. This result is in goodagreement with a previous study using a Ni/g-Al2O3 catalystreported by Newnham et al.40 They stated that a higher initialCH4 conversion was achieved when using the impregnatedsamples, owing to the presence of Ni particles on the externalsurface of the catalysts. In addition, some hypotheses suggestedthat CO2 is adsorbed at basic sites in the metal–support inter-face. Thus, the CO2 conversion could be closely related to thebasicity of the catalyst.41 From the basicity study results shownin Fig. 8, it can be suggested that the number of basic sites ismore important compared to the strength of the basic sites.Eventhough Ni/MCM-41 possessed slightly stronger basic sitesthan Ni/MSN, the greater number of weaker basic sites in theNi/MSN resulted a higher CO2 conversion (Fig. 9B).

Fig. 9C shows the relationship of H2/CO ratio towards thereaction temperature. At 500 �C, the Ni/MSN and Ni/MCM-41catalysts showed H2/CO ratios of 0.80 and 0.79, respectively.Although H2 and CO are formed simultaneously according tothe stoichiometry of CO2 reforming of CH4, an excess of CO withrespect to H2 was detected. This is probably due to the occur-rence of RWGS reactions that consume the produced H2 andcontributed to the production of CO, which would then lowerthe H2/CO ratio.42 As the temperature increased to 600 �C, theH2/CO ratio was observed to be greater than 1. This indicatedthat at higher temperatures, the CH4 cracking reactions wereprevalent. This was consistent with previous thermodynamicsstudies, in which increasing reaction temperatures favored theformation of H2 through various reactions, such as the Bou-douard reaction and CH4 cracking.43 The H2/CO ratio graduallyapproaches the stoichiometric value of unity as the temperatureincreases up to 800 �C, implying a balance between theproduction and consumption of H2 and CO. It is desirable toobtain a H2/CO ratio between 0.8 and 2.6, as this is the mostappropriate for upstream processing. The H2/CO ratio (0.80–1.2) produced by the presented CO2 reforming of CH4 is,therefore, more appropriate for upstream processing, as astoichiometric H2/CO ratio of 3 is obtained from the existingsteam reforming process.20 It is essential to choose 750 �C as theoptimal reaction temperature, considering the high catalyticactivity and appropriate H2/CO ratio.

The stability of Ni/MSN and Ni/MCM-41 catalysts for theCO2 reforming of CH4 reaction was studied at 750 �C. Theresults obtained for the CH4 conversion as a function of timeon stream (1800 min), are presented in Fig. 10. The catalyticactivity of Ni/MSN remains almost constant throughout theentire 1800 min on stream with a CH4 conversion of 92.2%. Incontrast, the conversion for Ni/MCM-41 started to declinefrom 89.7 to 85.2% aer 600 min time on stream, which may

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be attributed to the presence of a carbon deposit on thesurface of the catalyst. Therefore, a regeneration process wasperformed at 750 �C with 30 min of air and 1 h of H2 to removeany deposited carbon from the surface of the catalyst. Thecatalytic activity of Ni/MCM-41 was recovered aer regenera-tion; however, a slow but apparent deactivation was observedwith increasing time on stream aer reacting for 720 min.Aer a second regeneration, the change in the catalyticbehavior of Ni/MCM-41 followed the same tendency as it didaer the rst regeneration, but the values were obviouslydecreased. The deactivation behavior of Ni/MCM-41 resultsaround 8% of total loss in CH4 conversion. It is important tonote that the stable activity of the Ni/MSN was still observedaer a time on stream of 1800 min.

Characterization of spent Ni/MSN and Ni/MCM-41 catalysts

In order to clarify the origin of the deactivation of Ni-modiedcatalysts, the spent Ni/MSN and Ni/MCM-41 catalysts aer1800 min on stream at 750 �C were characterized by TGA, TEM,and XRD. TGA-DTA analysis was used to estimate the carboncontent and species of the spent Ni/MSN and Ni/MCM-41catalysts, and the results are shown in Fig. 11A. The weightloss was estimated from 110 �C to exclude any interference ofmoisture. Two exothermic peaks were observed for both spentNi/MSN and Ni/MCM-4 from the DTG proles, suggesting thattwo different types of carbon were deposited. A broad peakcentered at 300 �C and 700 �C could be attributed to theoxidation of amorphous carbon and coke deposits withdifferent degrees of graphitization, respectively.44 A similaramount of carbon (<3%) appeared to be deposited on both theNi/MSN and Ni/MCM-41.

In order to get insight into the type of carbon on the surfaceof spent Ni/MSN and Ni/MCM-41, TEM analysis was employed.Fig. 11B shows TEM images of spent Ni/MSN and Ni/MCM-41.Two different types of carbon were formed on spent Ni/MSNand Ni/MCM-41, which consisted of carbon nanotubes andencapsulating carbon (shell-like). Carbon deposit on spent Ni/MSN was mostly in the form of carbon nanotubes with ahollow internal channel, whereas shell-like carbon was mainlyformed on spent Ni/MCM-41. It is not surprising that the Ni/MSN catalyst exhibited a very stable catalytic performance in

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Fig. 11 (A) TG-DTG curves, (B) TEM images and (C) XRD patterns of (a)reduced Ni/MCM-41, (b) spent Ni/MCM-41, (c) reduced Ni/MSN, and(d) spent Ni/MSN catalysts.

Paper RSC Advances

the stability evaluation, because the Ni nanoparticles at the tipof carbon nanotubes would still be available and active for CH4

decomposition. However, the surface coverage of shell-likecarbon on Ni/MCM-41 would encapsulate and subsequentlyreduced the accessibility of the Ni nanoparticles to CH4 mole-cules, leading to catalyst deactivation.

The XRD pattern for the reduced and spent Ni/MSN and Ni/MCM-41 catalysts are shown in Fig. 11C. Three new peakscorresponding to metallic Ni were observed at 2q ¼ 44.6, 51.8,and 77.0� for the catalysts, suggesting that all Ni nanoparticlesare in the metallic state aer the reaction. No evidence couldbe found for any possible compound formation between Niand carbon or silica. The Ni/MSN signal exhibited a narrowerpeak width for the Ni (111) diffraction, which indicates thepresence of larger Ni nanoparticles. The average crystallitesizes of the metallic Ni particles calculated using Scherrerequation were increased from 4.2 to 22 nm for Ni/MSN and 2.5to 17 nm for Ni/MCM-41 aer stability test. The smaller Nicrystallite sizes in Ni/MCM-41 may be attributed to theconnement of the Ni nanoparticles in the pores of MCM-41,which prevents the agglomeration or sintering of the Ninanoparticles. This result suggested that Ni sintering is notthe main factor in catalyst deactivation. Moreover, a new peakappeared at 2q ¼ 26.1� for the spent Ni/MSN, which could beassigned to carbon deposits with a graphitic nature. Recallingthat carbon deposits were detected in the TGA for Ni/MCM-41,the absence of graphite diffraction at 2q ¼ 26.1� indicates thatcarbon was most probably dispersed on the surface of the Ninanoparticles inside the MCM-41 pores.

As shown in stability and regeneration study, Ni/MSNshowed a stable performance for 1800 min and Ni/MCM-41showed a partial deactivation aer 600 min time on stream.The regeneration of Ni/MCM-41 at 750 �C with air and hydrogendid not recover the catalytic performance and the activitycontinued to decrease. Characterization of the Ni/MSN and Ni/MCM-41 catalysts aer the rst regeneration (refer ESI Fig. S2†)indicated that the deactivation of Ni/MCM-41 was likely to becaused by coke accumulation. Eventhough carbon nanotubeswere observed on the surface of spent Ni/MSN, the regenerationprocess successfully removed the deposited carbon. On theother hand, the carbon formed on the surface of Ni/MCM-41

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seemed to be more stable and resistant to oxidation, thuscontinued to deactivate the catalyst. Basically, the carbonspecies formed both on the support and on the metal can beremoved by reaction with adsorbed surface CO2 and adsorbedoxygen species.45 The use of supports with a large number ofbasic sites favors the adsorption and dissociation of CO2, hencecontributes towards the gasication of carbonaceous depositsand prevents deactivation through coke formation.46 In thisstudy, the ability of Ni/MSN to resist the formation of deacti-vating carbon deposits compared to Ni/MCM-41 may be relatedto its basicity. The presence of large number of basic sitesresulted from the presence of interparticles voids led to lowercoke deposition rates and enhanced the catalyst activity andstability.47 The carbon nanotubes appeared on the surface of Ni/MSN can be easily remove aer regeneration of the catalyst.Therefore, it is suggested that basicity in Ni/MSN is appropriatein attenuating deactivating carbon formation, thus enhancingthe stability of the reaction.

Conclusions

Ni supported on MSN and MCM-41 were successfully preparedusing an in situ electrochemical method. The ndings havedemonstrated that a higher and more stable catalytic activitycould be achieved with Ni loaded on a MSN support than Ni/MCM-41. The characterization results indicated that thedifferent locations of Ni nanoparticles deposited on MSN andMCM-41 could be attributed to the different size of the pores inthe supports. H2-TPR and IR spectra of adsorbed CO indicatedthat Ni located on the interparticles surface of MSN was moreeasily reduced and was accountable for the higher CH4

conversion. Meanwhile, the higher CO2 conversion on Ni/MSNwas caused by the higher basicity, as evidenced from thepyrrole adsorbed IR spectroscopy. In this study, the synergisticeffect of Ni location and catalyst basicity was found to be the keyfactor in the enhanced CO2 reforming of CH4 performance overNi/MSN. The XRD and TEM analyses of the spent catalystsindicated that sintering of Ni nanoparticles did not cause thedeactivation, but the stability of the reaction was controlled bythe type of carbon deposited on the catalyst. A correlationbetween basicity and type of coke deposit on the surface ofcatalyst was observed in the stability evaluation. Ni/MSNshowed a stable performance for a time on stream of 1800min, as a result of the improved basicity, and consequentlyinhibited the formation of deactivating shell-like carbon. Thisstudy showed the potential use of Ni/MSN prepared by elec-trochemical method in the CO2 reforming of CH4.

Acknowledgements

This work is supported by the Universiti Teknologi Malaysiathrough Research University Grant no. 05H09. Our gratitudealso goes to the Ministry of Higher Education (MOHE) Malaysiafor the award of MyPhD Scholarship (Siti Munirah Sidik) andthe Hitachi Scholarship Foundation for the Gas ChromatographInstrument Grant.

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Notes and references

1 W. Chen, G. Zhao, Q. Xue, L. Chen and Y. Lu, Appl. Catal., B,2013, 136, 260.

2 D. Baudouin, U. Rodemerck, F. Krumeich, A. Mallmann,K. C. Szeto, H. Menard, L. Veyre, J.-P. Candy, P. B. Webb,C. Thieuleux and C. Coperet, J. Catal., 2013, 297, 27.

3 Z. Wang, X.-M. Cao, J. Zhu and P. Hu, J. Catal., 2014, 311,469.

4 S. Ozkara-Aydinoglu and A. E. Aksoylu, Catal. Commun.,2010, 11, 1165.

5 Z. Liu, J. Zhou, K. Cao, W. Yang, H. Gao, Y. Wang and H. Li,Appl. Catal., B, 2012, 125, 324.

6 A. A. Jalil, N. Kurono and M. Tokuda, Synlett, 2001, 12, 1944.7 A. A. Jalil, N. Kurono and M. Tokuda, Synthesis, 2002, 18,2681.

8 S. Triwahyono, A. A. Jalil, R. R. Mukti, M. Musthofa,N. A. M. Razali andM. A. A. Aziz, Appl. Catal., A, 2011, 407, 91.

9 N. H. N. Kamarudin, A. A. Jalil, S. Triwahyono, R. R. Mukti,M. A. A. Aziz, H. D. Setiabudi, M. N. M. Muhid andH. Hamdan, Appl. Catal., A, 2012, 431, 104.

10 M. A. A. Aziz, A. A. Jalil, S. Triwahyono and M. W. A. Saad,Chem. Eng. J., 2014, 260, 757.

11 L. Li, S. He, Y. Song, J. Zhao, W. Ji and C.-T. Au, J. Catal.,2012, 288, 54.

12 A. H. Karim, A. A. Jalil, S. Triwahyono, S. M. Sidik,N. H. N. Kamarudin, R. Jusoh, N. W. C. Jusoh andB. H. Hameed, J. Colloid Interface Sci., 2012, 386, 307.

13 N. H. N. Kamarudin, A. A. Jalil, S. Triwahyono,N. F. M. Salleh, A. H. Karim, R. R. Mukhti, B. H. Hameedand A. Ahmad, Microporous Mesoporous Mater., 2013, 180,235.

14 M. Iwamoto, Y. Tanaka, J. Hirosumi, N. Kita andS. Triwahyono, Microporous Mesoporous Mater., 2001, 48,271.

15 N. Wang, W. Chu, T. Zhang and X. S. Zhao, Int. J. HydrogenEnergy, 2012, 37, 19.

16 D. Liu, R. Lau, A. Borgna and Y. Yang, Appl. Catal., A, 2009,358, 110.

17 U. Izquierdo, V. L. Barrio, K. Bizkarra, A. M. Gutierrez,J. R. Arraibi, L. Gartzia, J. Banuelos, I. Lopez-Arbeloa andJ. F. Cambra, Chem. Eng. J., 2014, 238, 178.

18 L. Xu, Z. Miao, H. Song and L. Chou, Int. J. Hydrogen Energy,2014, 39, 253.

19 P. Djinovic, J. Batista and A. Pintar, Int. J. Hydrogen Energy,2012, 37, 2699.

20 E. Lovell, Y. Jiang, J. Scott, F. Wang, Y. Suhardja, M. Chen,J. Huang and R. Amal, Appl. Catal., A, 2014, 437, 51.

21 N. Wang, X. Yu, Y. Wang, W. Chu and M. Liu, Catal. Today,2013, 212, 98.

22 M. Selvaraj, P. K. Sinha, K. Lee, I. Ahn, A. Pandurangan andT. G. Lee, Microporous Mesoporous Mater., 2005, 78, 139.

37414 | RSC Adv., 2015, 5, 37405–37414

23 M. Selvaraj, B. H. Kim and T. G. Lee, Chem. Lett., 2005, 34,1290.

24 A. A. Jalil, M. A. H. Satar, S. Triwahyono, H. D. Setiabudi,N. H. N. Kamarudin, N. F. Jaafar, N. Sapawe andR. Ahamad, J. Electroanal. Chem., 2013, 701, 50.

25 N. Sapawe, A. A. Jalil, S. Triwahyono, S. H. Adam, N. F. Jaafarand M. A. H. Satar, Appl. Catal., B, 2012, 125, 311.

26 X. S. Zhao, G. Q. Lu, A. K. Whittaker, G. J. Millar andH. Y. Zhu, J. Phys. Chem. B, 1997, 101, 6525.

27 Z. Qin, B. Shen, Z. Yu, F. Deng, L. Zhao, S. Zhou, D. Yuan,X. Gao, B. Wang, H. Zhao andH. Liu, J. Catal., 2013, 298, 102.

28 N. W. C. Jusoh, A. A. Jalil, S. Triwahyono, H. D. Setiabudi,N. Sapawe, M. A. H. Satar, A. H. Karim,N. H. N. Kamarudin, R. Jusoh, N. F. Jaafar, N. Salamunand J. Efendi, Appl. Catal., A, 2013, 468, 276.

29 X. S. Zhao and G. Q. Lu, J. Phys. Chem. B, 1998, 102, 1556.30 K. Tao, L. Shi, Q. Ma, D. Wang, C. Zeng, C. Kong, M. Wu,

L. Chen, S. Zhou, Y. Hu and N. Tsubaki, Chem. Eng. J.,2013, 221, 25.

31 K. A. Layman and M. E. Bussell, J. Phys. Chem., 2004, 108,10930.

32 M. Mihaylov, K. Hadjiivanov and H. Knozinger, Catal. Lett.,2001, 76, 59.

33 S. Lim, C. Wang, Y. Yang, D. Ciuparu, L. Pfefferle andG. L. Haller, Catal. Today, 2007, 123, 122.

34 J. Kucera and P. Nachtigall, J. Phys. Chem. B, 2004, 108,16012.

35 M. A. A. Aziz, A. A. Jalil, S. Triwahyono, R. R. Mukhti,Y. H. Tauq-Yap and M. R. Sazegar, Appl. Catal., B, 2014,147, 359.

36 L. Xiancai, W. Min, L. Zhihua and H. Fei, Appl. Catal., A,2005, 290, 81.

37 J. C. Lavalley, Catal. Today, 1996, 27, 377.38 B. Camarota, Y. Goto, S. Inagaki and B. Onida, Langmuir,

2011, 27, 1181.39 D. C. Carvalho, H. S. A. de Souza, J. M. Filho, A. C. Oliveira,

A. Campos, E. R. C. Milet, F. F. de Sousa, E. P. Hernandezand A. C. Oliveira, Appl. Catal., A, 2014, 473, 132.

40 J. Newnham, K. Mantri, M. H. Amin, J. Tardio andS. K. Bhargava, Int. J. Hydrogen Energy, 2012, 37, 1454.

41 T. Huang, W. Huang, J. Huang and P. Ji, Fuel Process.Technol., 2011, 92, 1868.

42 J. Z. Luo, Z. L. Yu, C. F. Ng and C. T. Au, J. Catal., 2000, 194,198.

43 Y. Li, Y. Wang, X. Zhang and Z. Mi, Int. J. Hydrogen Energy,2008, 33, 2507.

44 M. Khavarian, S. P. Chai and A. R. Mohamed, Chem. Eng. J.,2014, 257, 200.

45 D. Pakhare, V. Schwartz, V. Abdelsayed, D. Haynes,D. Shekhawat, J. Poston and J. Spivey, J. Catal., 2014, 316, 78.

46 J. H. Bitter, K. Seshan and J. A. Lercher, J. Catal., 1997, 171,279.

47 Y. H. Guo, C. Xia and B. S. Liu, Chem. Eng. J., 2014, 237, 421.

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